Pullability Range of Crystal for Failsafe Device - KBA82260

Question: What are the recommended pullable crystals to use with CY23FS08/CY23FS04? Does Cypress have any pullability range data for the recommended crystals?

 

Answer:

The table below shows the recommended crystals to use with CY23FS08/CY23FS04 and their measured pull range data:


Table 1: Measured Pull Range Data for XTAL with C0/C1 Ratio of 250
 

                                                                                                                                                                                                                                                                                                                                                                                                                 
    XTAL SN.    XTAL vendor (Ecliptek) part#    XTAL Freq. (MHz)    XTAL C0/C1 Ratio    Lower Pull Freq (MHz)    Higher Pull Freq (MHz)    Lower Pull Range (ppm)    Higher Pull Range (ppm)
    1    ECX-5712-19.5312M    19.5312    250    19.5272    19.5385    –204.80    373.76
    2    ECX-5712-19.5312M    19.5312    250    19.5271    19.5387    –209.92    384.00
    3    ECX-5712-19.5312M    19.5312    250    19.5272    19.5386    –204.80    378.88
    1    ECX-5712-19.200M    19.2    250    19.1957    19.2075    –223.96    390.63
    2    ECX-5712-19.200M    19.2    250    19.1955    19.2072    –234.38    375.00
    1    ECX-5712-15.625M    15.625    250    15.6213    15.6315    –236.80    416.00
    2    ECX-5712-15.625M    15.625    250    15.6214    15.6315    –230.40    416.00
    3    ECX-5712-15.625M    15.625    250    15.6214    15.6315    –230.40    416.00
    4    ECX-5712-15.625M    15.625    250    15.6215    15.6300    –224.00    320.00
    Average Pull Range in PPM    –222.16    385.59

Experimental Calculation and setup:


To measure the pullability range of the digitally controlled crystal oscillator (DCXO) crystal, Cypress used the following equipment and parameters:

  •   A failsafe evaluation board is included with the CY23FS08 device
  •   Power supply voltages: VDDA, VDDB, and VDDC at 3.3 V
  •   Frequency select pins: S[1:4]: 1001
  •   DCXO: Ecliptek XTAL at various frequencies (as mentioned in Table 1)

The above described setting of the frequency select pins sets Output/XTAL and REF/XTAL ratio to 1. Several set of DCXO’s with different fundamental frequencies and known C0/C1 (250) ratio were used.


To conduct the range test, following steps were used:


  1.   For a particularCrystal, the reference frequency was set equal to the Crystal frequency. This caused the FAIL#/SAFE signal (Pin 16 output of CY23FS08) to go High, indicating the DCXO was locked to the reference frequency.
  2.   Reference frequency is slowly changed (increased or decreased) until the FAIL#/SAFE signal (Pin 16 output of CY23FS08) went Low, indicating the point where the reference frequency passes beyond the pull range of the DCXO.

The frequency range, during which the DCXO remains locked to the external reference frequency (FAIL#/SAFE signal as High) gives the pullability range.


Cypress repeated this measurement for Crystal with different operating frequencies, achieving the results shown in Table 1. This pull range was then converted to ppm.


Theoretical Calculation with an Example


The pullability range can also be calculated theoretically. For a DCXO crystal, the frequency delta (higher or lower) achievable from its nominal frequency varies by the capacitive load (CL). The range from highest to lowest frequency is called the pullability range. To calculate the pullability range, use the equations:


Pullability Range = (fCLOADMIN – fCLOADMAX) / fCLOADMIN = (C1 / 2) * [(1 / (C0 + CLOADMIN)) – (1 / (C0 + CLOADMAX))]
Pullability Range in ppm = (C1 / 2) * [(1 / (C0 + CLOADMIN)) – (1 / (C0 + CLOADMAX))] * 106


For example, assume a stray capacitance of 7 pF and tuning capacitance range of 0 to 24 pF for the failsafe device. Use a CL (min) value of 7 pF and CL (max) value of 31 pF. The data has been normalized to the nominal XTAL frequency at the CL of 16 pF. Based on these values, pull range for the XTAL with C0/C1 ratio of 250 is calculated and given below in the table:


Table 2: Calculated Pull Range Data


                                                   
    C0/C1 Ratio    CL (min)    CL (max)    Pullability Range
    250    7    31    –240.27    391.30